The present invention relates to a method of manufacturing a semiconductor device and, more particularly, to a method of manufacturing a semiconductor device with a gate length or line width that has been reduced to the order of 0.1 to 0.2 .mu.m.
With recent tendencies toward an increasingly advancing information-oriented society, the milliwave band in which frequencies are 30 GHz and higher has been expected to find application in a multimedia mobile telecommunications system, a radio LAN, an automotive collision avoidance radar, and the like. To provide an ultra-high frequency device using the milliwave band as its operating frequency range, it is essential to reduce the gate length. Specifically, it is necessary to implement a method of forming a gate with a length on the order of 0.1 to 0.2 .mu.m. However, since gate resistance increases with a reduction in gate length, such a tendency toward a reduced gate length may cause the lowering of gain in the high frequency band and the degradation of noise immunity. As means for implementing both a reduced gate length and lowered gate resistance, a gate structure having a so-called T-shaped or mushroom-shaped configuration is effective, in which the lower part of a gate in contact with a substrate surface is scaled down and the upper part thereof is increased in cross-sectional area. The T-shaped gate structure is used widely in field-effect transistors (FETs) for ultra-high-frequency applications.
Referring to the drawings, a description will be given to a method of manufacturing a conventional semiconductor device having a T-shaped gate electrode. In the present application, the upper part of a T-shaped gate electrode having a T-shaped configuration which is formed relatively large for lower gate resistance is termed a "head portion" and the lower part of the T-shaped gate electrode which is formed relatively small for a shorter gate length is termed a "leg portion".
FIGS. 8(a) to 8(d) and FIGS. 9(a) to 9(c) show cross-sectional structures illustrating the individual process steps of a method of manufacturing a T-shaped gate electrode using electron beam (hereinafter referred to as EB) exposure for a conventional multi-layer resist. The example shown here uses typical polymethyl methacrylates (hereinafter referred to as PMMAs), which are different in sensitivity and formed in two layers.
First, as shown in FIG. 8(a), a lower-layer resist film 102 composed of the PMMA having a higher molecular weight and a lower sensitivity and an upper-layer resist film 103 composed of the PMMA having a lower molecular weight and a higher sensitivity are applied sequentially onto a semiconductor substrate 101. Then, as shown in FIG. 8(b), a first session of EB exposure is performed with respect to a region 103a of the upper-layer resist film 103 to be formed with the head portion of the gate electrode. As shown in FIG. 8(c), an opening 103b is formed in the upper-layer resist film 103 by developing the upper-layer resist film 103 and thereby removing the resist from the region 103a to be formed with the head portion.
Next, as shown in FIG. 8(d), a second session of EB exposure is performed with respect a region 102a of the lower-layer resist film 102 to be formed with the leg portion of the gate electrode. Then, as shown in FIG. 9(a), an opening 102b is formed in the lower-layer resist film 102 by removing the resist from the region 102a to be formed with the leg portion. As a result, a resist pattern for forming the T-shaped gate electrode is obtained in the opening 103b of the upper-layer resist film 103 and in the opening 102b of the lower-layer resist film 102.
Next, as shown in FIG. 9(b), a metal film 104A is vapor deposited over the entire surface of the semiconductor substrate 101. Subsequently, as shown in FIG. 9(c), the upper-layer and lower-layer resist films 103 and 102 are lifted off to provide the T-shaped gate electrode 104B formed of the metal film 104A.
However, the foregoing conventional method of manufacturing a semiconductor device having the T-shaped gate electrode has the following two problems.
First, since EB exposure is used for the exposure of the resist, equipment for EB exposure requires a tremendous investment, while the throughput thereof is low.
Second, the determinants of the gate length in the resist process are the widths of the openings in the resists composed of the PMMAs. However, if dry etching is performed for the purpose of preventing repellence of water during wet etching or the like, the gate length is increased disadvantageously due to a high etching rate and to low dry etching resistance of the PMMAs. Moreover, since the PMMAs have low heat resistance, the vapor deposition of metal on the PMMAs causes thermal deformation at the openings of the pattern.